Process



' mercaptoacids.

cultles.

to provide a satisfactory method of preparing Patented Aug. 27, 1946PROCESS Mark Wendell Farlow, Wilmington, DeL, assignor to E. I. du Pontde Nemours & Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application November 11, 1943, Serial No. 509,897

1 This invention relates to alpha-amino-beta- More particularly,itrelates to a process for the preparation of these acids and of thecorresponding disulfides.

The alpha amino beta-mercaptoacids and the corresponding disulfides, ofwhich cystein and cystine are, respectively, the best knownrepresentatives, are extremely important pharmaceutically because oftheir physiological activity. Methods for preparing these compounds areknown, but they involve, in general, a number of complicated steps andthe yields are not satisfactory.

An object of this invention is to provide a general method ofpreparation of alpha-aminobeta-mercaptoacids and the correspondingdisulfldes. Another object is to provide a twostep process for preparingthese compounds in good yields with a minimum of technical difll- Aparticular object of this invention is cysteine and cystine. pearhereinafter.

These objects are accomplished by the process of this invention, whichcomprises the steps of adding a thiocarboxylic acid to an alphaacylaminoalpha, beta-ethylenically unsaturated carboxylic acid or a functionalderivative thereof which on hydrolysis reverts to the acid, e. g., asalt, amide, ester, nitrile, acid halide or acid anhydride, andhydrolyzing the resulting alpha acylamino beta (acylthio) carboxylicacid or functional derivative to the correspondingalpha-amino-beta-mercaptocarboxylic acid, or directly to thecorresponding p,fl-diaminoflfl'dicarboxydialkyl disulfide if oxidizingconditions are used during the hydrolytic step.

The process may be illustrated by the following equations, which depictthe synthesis of Other objects will apcystine from thiolacetic acid andalpha-acetylaminoacrylic acid:

Claims. (Cl. 260-534) 1 In practice, steps 2 and 3 may be carried outseparately or combined, 1.e., the hydrolysis may be carried out underoxidizing conditions, to

- yield cystine directly.

Cal

The more detailed practice of the invention is illustrated by thefollowing example, wherein parts given are by weight. There are, of

course, many forms of the invention other than this specific embodiment.

Example To a mixture of 17.8 parts of alpha-acetylaminoacrylic acid(which may be prepared acc rding to the method described by Bergmann andGrafe in Z. physiol. Chem. 187, 191, (1930)) and parts of thiolaceticacid is added approximately 0.1 part of ascaridole. The mixture isrefluxed to complete solution, which requires about twenty minutes, thenfor an additional fifteen minutes. Evaporation of the reaction mixtureunder reduced pressure gives a crystalline residue which isrecrystallized from a mixture of chloroform and petroleum ether. Thereis obtained 241 parts (85% of the theoretical yield) of opticallyinactive N,S- diacetylcysteine (cf. Neuberger, Biochem. J. 32, 1455(1938)) melting at 118 C. The product has a neutralization equivalent of203, as compared with the theoretical value of 205.

FromN,S-diacetylcysteine, cystine is obtained as follows:

To a gently boiling solution of 10.3 parts of N,S-diacetylcysteine in120 parts of concen-- trated hydrochloric acid is added in smallportions, and as fast as it is decolorized, a 0.53 N

solution of. iodine in methanol, until the yellow iodine color remainsfor five minutes after the last addition. Somewhat over the calculatedquantity of iodine solution is required and the addition takesapproximately thirty minutes. The solution is then evaporated nearly todry ness, diluted with water, and soduim acetate added until thesolution is alkaline to Congo red. Crystalline cystine separates slowlyfrom the solution. It is removed by filtration and washed with water,alcohol and ether. There is obtained 4.3 parts (72% of the theoreticalyield) of dry cystine. Analysis: Calculated for CBH12N2O4S22 C, 30.4%:H, 5.4%; N, 11.0%; S, 26.4%. Found: C, 30.0%; H, 5.0%; N. 11.7; S,26.6%.

In the above method, it is obviously possible to isolate, cysteine assuch, if desired. This is done by carrying out the hydrolysis undernonoxidizing conditions, for example by warming with dilute hydrochloricacid and evaporating to dryness, giving cysteine hydrochloride fromwhich csysteine is isolated by neutralization in the usual manner. v

The invention has been illustrated by reference I to cysteine andcystine. It is, however, applicable to the preparation of any desiredaliphatic alphaamino-beta-mercaptocarboxylic acid or functionalderivative thereof which revertsto the acid on hydrolysis. For example,in the first step of the method, other carboxylic acids, such asthiolpropionic, thiolbutyric and thiolbenzoic acids may be used. Thereis, however, little advantage of orvariation therefrom which conforms tothe in using rare and expensive thiol acids since the acylradical ishydrolyzed ofi during the second step. In place ofalpha-acetylaminoacrylic acid, there may be used any desiredalpha-acylaminoalpha, beta-ethylenically unsaturated carboxylic acid andfunctional derivatives thereof such as, for example,alpha-propionylaminoacrylic acid, sodium alpha-acetylaminoacrylate,alpha-acetylaminomaleic acid, ethyl alpha-benzoylaminoacrylate,alpha-stearoyl-aminoacrylamide, alphalauroylaminocrotonic acid,alpha-butyrylaminocinnamic acid, alpha-acetylaminoacrylonitrile, alphaacetylaminoacrylic anhydride, alpha caprylylaminobeta,beta-dimethylacrylic acid, alpha-acetylaminoundecylenic acid,alpha-acety aminotetrahydrofurylacrylic acid etc. Here again, the mosteconomical acylamino group is preferably used since the acyl radical ishydrolyzed ofl during the second step. In the case of certain functionalderivatives of the alphaacylamino-alpha, beta-unsaturated acids, e. g.,the chloride, an excess of the thiolcarboxylic acid may be needed. Thisreacts with th functional groups but is hydrolyzed off later.

The alpha acylamino alpha, beta ethylenic acids are sometimes assignedalternative formulas asshown below:

where R, R and R." are hydrogen or substituents. The two types'aretautomeric, hence compounds represented by either formula are suitablestarting materials in the process of this invention.

A catalyst for the addition of the thiol acid to thealpha-acylamino-alpha, beta-ethylenic acid is not necessary, though itfacilitates the reaction.

Addition-promoting catalysts such as benzoyl peroxide, ascaridole, etc.are satisfactory for this purpose. h

The reaction may be carried out in an inert solvent such as benzene orether, in which case an excess of thiol acid, or of functionalderivative thereof, is unnecessary,.or, as in the example, an excess ofthiol acid may be used as solvent. When either the reaction mixture orthe reaction product is liquid, neither a solvent nor an excess of thiolacid is required. The reaction sometimes take place at room temperature,but mild heating accelerates the addition and-is therefore advantageous.'The choice of the proper conditions for any given set of reactantsoiTers no difliculties to those skilled in the art.

In the second step of the process, any convenient hydroyzing agent maybe used, such as mild alkalies, e. g., alkali carbonates or alkalineearth oxides, acids, etc., but neutral or acidic conditions are muchpreferred because of the relative instability of the products towardalkalies. The hydrolysis may be carried out either under non-oxidizingconditions, if it is'desired to isolate thealpha-amino-beta-mercaptoacid, or

spirit of the invention is intended to be included within the scope ofthe claims.

What is claimed is: h 1. Process which comprises reacting athiolcarboxylic acid with a member of the class consisting ofalpha-acylamino-alpha, beta-ethylenically unsaturated carboxylic acidsand functional derivatives thereof which revert to the acid onhydrolysis and hydrolyzing the addition product to the correspondingalpha-amino-beta-mercapto carboxylic acid.

2. Process which comprises reacting a thiolcarboxylic acid with a memberof the class consisting of alpha-acylamino-alpha, beta-ethylenicallyunsaturated carboxylic acids and functional derivatives thereof whichrevert to the acid on hydrolysis and hydrolyzing the additio product tothe corresponding alpha-amino-beta mercapto carboxylic acid in an acidicaqueous medium.

3. Process which comprises reacting a thiolcarboxylic acid with a memberof the class consisting of alpha-acylamino-alpha, beta-ethylenicallyunsaturated carboxylic acids and functional derivatives thereof whichrevert to the acid on hydrolysis and hydrolyzing the'addition product inan acidic aqueous medium under oxidizing conditions to the correspondingp,p-diamino-p,p'-dicarboxydialkyl disulfide.

4. Process for the preparation of alpha-acylamino-beta-acylthiomonocarboxylic compounds which comprises reacting a thiolcarboxylic acidwith a member of the class consisting of alphaacylamino-alpha,beta-ethylenically unsaturated carboxylic acids and functionalderivatives thereof which revert to the acid on hydrolysis.

5. Process which comprises reacting thiolacetic acid with a member ofthe class consisting of alpha-acetylamino-alpha, beta-ethylenicallyunsaturated carboxylic acids and functional derivat ves thereof whichrevert to the acid on hydrolysis and hydrolyzing the addition product tothe corresponding valpha-amino-beta-mercapto carboxylic acid.

6. Process which comprises reacting thiolacetic acidic aqueous mediumunder oxidizing conditions to the correspondingp,p'-diamino-p,p'-dicarboxydialkyl disulfide.

8. Process for the preparation of alpha-acetylamino-beta-acylthiomonocarboxylic compounds which comprises reacting thiolacetic acid witha member of the class consisting of alpha-acetyl- 10. Process whichcomprises reacting thioiacetic acid with alpha-acetyiaminoacrylic acidand hydrolyzing the alpha-acetylamino-beta-a.cetylthiopropionic acidunder oxidizing conditions in 5 an aqueous acid medium to cystine.

MARK WENDELL minnow.

